Remove UserVisibleFeatureKey from MqlsFeatureMetadata.

This was only used to verify that the feature was enabled in the UI before logging its usage. But if we have a log entry for the feature, that means it should have been enabled to start with.

Removing to simplify code, but also onboarding process since there was no easy way to make sure that users of MqlsFeatureMetadata would remember to go back and add the UserVisibleFeatureKey once they integrate with the chrome://settings/AI UI.

Bug: 373948487
Change-Id: I3ab2cf418b25e6e65a6e9cb178527d2158ed54b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5939932
Reviewed-by: Steven Holte <holte@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Simon Pelchat <spelchat@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1370283}